This invention was made with government support under Grant Number DMI-9702913 awarded by the National Science Foundation. The government has certain rights in the invention.
This invention relates to fixturing systems for workpieces which are to be machined, and more particularly to a method and apparatus for encapsulating a workpiece to be machined to form an encapsulation block of standardized size and shape so as to facilitate universal automated fixturing for workpieces of varying sizes and shapes.
Machined parts can have complex shapes and can include thin and fragile areas. Some of the parts are also quite small. All of these factors complicate machining of the parts, and in particular the fixturing of the parts to facilitate such machining. Proper fixturing of a part for machining generally involves a number of factors including:
(a) Immobilization of the workpiece to allow aggressive machining of the part without spurious movement thereof. Without proper immobilization, machining must be done gently in order to avoid spurious movement of the part during machining, which movement can result in the destruction of the part. This significantly increases the time required to complete a machining operation.
(b) Unobstructed machining paths. Obstructions in the machining path significantly complicate the generation of tool paths and other computer aided manufacturing (CAM) data for the manufacturing process, thereby increasing both the time and expense for manufacturing the part.
(c) Locate the workpiece with absolute accuracy, particularly where parts are being manufactured with very fine tolerances. Even a small fixturing error in locating the part for a machining operation can result in parts being out of tolerance, and therefore worthless.
(d) Support the workpiece entirely. This is a particular problem where a significant portion of the original workpiece is removed during the machining operation, leaving fragile points on the part which must be thoroughly supported to avoid cracking, bending or breaking during subsequent machining operations. Fixturing to fully support a partially machined part so as to prevent damage thereto is frequently a difficult and expensive procedure. At times, it is not possible to properly fixture such parts and it is therefore necessary to manufacture them in two or more pieces which must subsequently be secured together.
Achieving the above objectives is further complicated by the fact that many, if not most, parts need to be machined on all six surfaces of for example an originally rectangular workpiece, or on at least five such surfaces. Since fixturing to achieve the above objectives generally requires gripping and holding the workpiece on several surfaces, typically four or five surfaces, it is normally necessary to refixture a part several times during the machining thereof. Therefore, multiple fixtures are generally required for the machining of a single part, as many as six different fixtures being required in the extreme case.
However, the design of custom fixtures and the fabrication of such fixtures for a particular part is an expensive procedure. While this procedure can be justified where the cost is spread out over the manufacture of many thousands of parts, the cost of developing and fabricating fixtures can be prohibitive for custom parts having a small run and is a particular problem where parts are being prototyped or are being made for use in a prototype product.
A need therefore exists for a universal fixturing system which eliminates the need for designing and fabricating custom fixtures for each part, and in particular designing multiple custom fixtures for each part to accommodate machining on various sides thereof, while still meeting all of the fixturing requirements indicated above. A proposal for such a universal fixturing system is provided in xe2x80x9cA Methodology for Integrated CAD and CAM in Milling,xe2x80x9d a Ph.D. thesis of S.E. Sarma, University of California, Berkeley, 1995 and in xe2x80x9cReference Free Part Encapsulation: A New Universal Fixturing Concept,xe2x80x9d S. Sarma and P. Wright, Journal of Manufacturing Systems, Vol. 16/No. 1, 1997 (xe2x80x9cthe Sarma Papersxe2x80x9d), which discuss the concept of encapsulating a workpiece to be machined in a material having a lower melting temperature than that of the material for the workpiece, the encapsulated block containing the encapsulated workpiece having a known, standardized size and shape which fits in a standardized fixture. Further, the concept involves re-encapsulating any portions of the encapsulated block, including the workpiece embedded therein, which are machined away during the machining of a given side or sides. This re-encapsulation accomplishes two functions. First, it assures that the block being fixtured is always of standardized size and shape, which can be mounted in a standardized fixture and can be precisely located therein. Second, it assures that all portions of the workpiece, regardless of how heavily machined and how fragile, are fully supported within the encapsulation material so that machining may be performed aggressively without risking cracking or breaking of the part, and without requiring complicated specialized fixturing.
However, while the concepts presented in the Sarma Papers represent an interesting approach to the universal fixturing problem, there are many problems involved in implementing such an encapsulation and re-encapsulation process, and the Sarma Papers do not describe a practical system for performing the encapsulation process which overcomes these various problems. Thus, while the encapsulation technique discussed in the Sarma Papers represents a promising approach to universal fixturing of parts or other objects to be machined, a need exists for a practical implementation of this approach.
In accordance with the above, this invention provides a mold for use in encapsulating a workpiece of a material having a density D1 in an encapsulant having a density D2 to form an encapsulated block. The mold includes a structure defining four sidewalls of an encapsulation cavity in the mold, a top plate and a bottom plate, which plates mate with the structure on opposite sides thereof to fully define and seal the cavity. One of the plates is a gate plate having a plurality of openings formed therethrough and spaced over the area of the gate plate adjacent the cavity, with encapsulant being applied to the cavity through the gate plate openings. The openings may be substantially evenly spaced over the area of the gate plate; and are, in any event, sufficient in number and are positioned so that at least one opening overlies the smallest feature to be machined in the workpiece. The diameter at least at the cavity side of each of the gate plate openings is preferably no more than approximately 0.03 inches. Each of the openings preferably has a selected small diameter extending for a short distance from the cavity side of the opening and then flares to increasing diameters for its remaining length. The gate plate is preferably formed of a material having good high temperature characteristics but poor thermal conductivity, such material for example being a ceramic.
For one embodiment of the invention where D2 is greater than D1, the mold is oriented with the top and bottom plates being substantially horizontal, and an orienting structure is formed on the cavity side of the top plate, the workpiece floating up into orienting contact with such orienting structure when encapsulant is applied to the cavity. The orienting structure may for example be a depression formed in the top plate, which depression preferably has chaffered walls. The workpiece is preferably oriented in the cavity so that the thickness of encapsulant on the workpiece is substantially uniform. For preferred embodiments, the top plate is the gate plate. A recess having rounded corners may be formed in the bottom plate.
The cavity preferably has a substantially rectangular shape, with sidewalls that are substantially perpendicular to the top plate, to the bottom plate, and to each other. The structure portion of the mold may be divided into two L-shaped pieces, the parting line for such pieces bisecting the structure diagonally at two corners thereof. Each of the L-shaped pieces may (i) have a corner extending from top to bottom thereof; and (ii) be hinged at substantially the bottom of such corner along a line substantially parallel to the parting line. Each of the L-shaped pieces is normally biased so as to pivot about its hinge upward from the bottom plate and away from each other, the L-shaped pieces being moved down into sealing engagement with the bottom plate and with each other along the parting line when the top plate is pressed down toward the bottom plate against the L-shaped pieces. For these embodiments, the cavity may have a substantially cubic shape, so that an encapsulated block is reinsertable into the cavity for re-encapsulation in any orientation thereof that has sides of the block parallel to cavity walls.
For a second embodiment of the invention, the top plate is the gate plate, and the bottom plate has a plurality of projections extending therefrom into the cavity, the projections being encapsulated in the encapsulant with the workpiece to secure the bottom plate to the encapsulated block. For this embodiment, the sidewalls of the cavity, as defined by the structure, are preferably at a draft angle to facilitate removal of the block from the mold. The projections may include at least one feature, for example a screw thread, facilitating entrapment of the projection in the encapsulant.
For still another embodiment of the invention, the mold structure retains an encapsulated block therein after encapsulation, the structure including features facilitating mounting thereof for fixturing and/or re-encapsulation of the workpiece. For this embodiment, the structure preferably includes at least one feature facilitating retainment of the block therein.
The invention also includes a system for encapsulating and re-encapsulating a workpiece to be machined in an encapsulant to form an encapsulated block, the system including a source of melted encapsulant, a mold having the structure, top plate and bottom plate defining a cavity described above, a clamping mechanism which holds the structure and plate of the mold together under a selected pressure, and an injector mechanism which moves melted encapsulant under pressure from a source thereof through the gate plate and into the cavity. The clamping mechanism may include a reservoir for melted encapsulant, the injector mechanism applying pressure to melted encapsulant in the reservoir to force encapsulant therefrom through the openings in the gate plate to the cavity.
The source may include a storage tank having a heater for melting encapsulant in the tank and an outlet port connected by heated plumbing to the injector mechanism. The injector mechanism preferably includes a first check valve between the source and the injector mechanism which permits encapsulant flow only in the direction from the source to the injector mechanism, and for preferred embodiments also includes a second check valve between the injector and clamping mechanism which permits encapsulant flow only in a direction from the injector mechanism to the clamping mechanism. A bypass is preferably provided around the second check valve for pressure equalization in the system, and particularly in the clamping mechanism, once the injector mechanism is deactivated. The injector mechanism should also include a piston and at least one heater for maintaining the temperature in the injector mechanism above a melting temperature for the encapsulant. At least one high-temperature elastomer O-ring may be provided for sealing the piston.
The clamping mechanism may include a top support to which the mold top plate is mounted and a bottom support to which the mold bottom plate is mounted, the mold structure being mounted between the top and bottom plates, and a primary clamping stage mounted to move at least one of the supports towards the other when activated to seal the mold under pressure. For a preferred embodiment, the primary clamping stage includes a pneumatic piston and a mechanical enhancement for substantially increasing the force applied by such piston to the mold. The clamping mechanism may be adapted to clamp molds of different types and sizes and may include a secondary clamping stage for adjusting the position of at least one of the supports when the primary clamping stage is in its deactivated position to compensate for the type and size of mold being used. At least one locking block may be provided which is mounted between the bottom support and a housing member of the clamping mechanism for at least some of the mold types/sizes, the height of the block between the support and the housing member varying with the type/size of the mold. Each of the locking blocks may include a fine adjustment mechanism operable to compensate for small variations in mold size.
The clamping mechanism may also be designed to thermally isolate the mold from the clamping mechanism, the gate plate, for example, being of a material having poor thermal conductivity, and a cooling plate and insulation being provided between the bottom plate of the mold and a bottom support to both isolate and more quickly cool the mold once encapsulant has been injected.
A component may also be provided which is positioned adjacent the gate plate and through which encapsulant flows before reaching the gate plate, which component permits flow of encapsulant therethrough when the encapsulant is under pressure from the injector mechanism and which breaks the flow of encapsulant when such pressure is removed. For a preferred embodiment, the component is an elastomer diaphragm having a split form therein. A valve may also be provided ahead of the gate plate to selectively cut off encapsulant flow to the mold cavity even when pressure is being applied by the injector mechanism.
Quick release elements may also be provided for releasably securing at least one of the mold elements (i.e., the structure, top plate and bottom plate) in the clamping mechanism, the quick release elements facilitating removal of the block from the mold and the changing of molds. For a preferred embodiment, the top plate is the gate plate, and the quick release elements include a first set of such elements releasably securing the top plate to a top structure of the clamping mechanism, and may include a second set of elements for releasably securing the mold structure to the bottom plate, the bottom plate being mounted to a bottom structure of the clamping mechanism. Each quick release mechanism may include a piston-driven wedge shaft co-acting with a corresponding wedge-shaped recess formed in the mold structure/plate being clamped. For an alternative embodiment, the above is reversed, the structure being secured by elements to the top structure, also securing the gate plate thereto, and the bottom plate being secured to the bottom structure. The clamping mechanism and/or the injector mechanism are preferably pneumatically driven, as are the quick release elements.
The invention also includes a method for encapsulating and re-encapsulating a workpiece to be machined in an encapsulant, which method includes the steps of: providing a mold having an encapsulation cavity, the side walls of which are defined by a structure, and including a top plate and a bottom plate, which plates mate with the structure on opposite sides thereof to fully define and to seal the cavity, one of the plates being a gate plate having a plurality of openings through which encapsulant is applied to the cavity; placing the workpiece to be encapsulated in the cavity; clamping the structure and plates of the mold together under a selected pressure; and injecting melted encapsulant under pressure from a source thereof through the gate plate to the cavity to form an encapsulated block. The method may also include the steps of removing the encapsulated block from the mold; machining at least one surface of the block; re-inserting the block in the encapsulation cavity; and repeating the clamping and injecting steps to refill machined features, thus restoring the fully encapsulated block. The sequence of steps indicated above may be repeated a number of times sufficient for all sides of the workpiece which are to be machined to be machined. When machining on the workpiece is completed, the step of melting and removing encapsulant from the workpiece is performed. The bottom plate may have projections formed thereon which projections are embedded in the block during the injection step. For this embodiment, the removing step includes separating the bottom plate from the mold with the block affixed thereto, the block/workpiece being machined and being reinserted into the mold for re-encapsulation while affixed to the bottom plate. Alternatively, the structure of the mold may have features facilitating attachment of the structure to the block during the injection step, the structure remaining with the block during the removal, machining and reinsertion steps and being utilized to facilitate each of these steps.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention as illustrated in the accompanying drawings.